Yep, get a non-reserved ticket and ride two stops to Shin Yokohama. You could save a few pennies and ride from Shinagawa too.
I should mention that you probably won't break 70 kms on that stretch.
I don't care, I took it down to Kyoto last time. I don't want to spend the day riding trains this time. I won't tell if you won't We are going to have JR passes anyway so it's all covered. Alternatively can you suggest a desirable destination to go on the bullet, no more than 2 hours outside Tokyo?